There are many situations in which it is advisable to provide a point of weakness in a pipeline, or a bundle of pipelines, so that should some excessive external force act on the pipeline, it will be enabled to break at the point of weakness without causing damage to the installations to which the pipeline is attached. One particular environment in which danger of such an excessive force arises in an undersea oil pipeline connected to an undersea oil well or other similar equipment.
It has been proposed to provide a separable and at least partially pressure balanced safety pipeline connector or joint for this purpose in U.S. Pat. No. 4,059,288 issued 22 Nov. 1977 to Harvey O Mohr. This safety joint is designed with the purpose of providing a pressure balanced weak point, whereby the connector can separate at a predetermined tension load, for example if the pipeline is caught on a ship's anchor and dragged, wherein the pressure prevailing in the pipeline will have as small an effect as possible on the separation. This is achieved by having a housing one axial end of which is connected to the pipeline and the other axial end is left open, the joint including a pipe extension member having one axial end adapted for connection to the pipeline for transmission of line fluids therethrough and the other axial ends arranged to be telescopically received in the housing in sealed relationship thereto. The housing and extension member are arranged to provide an annular pressure chamber to which the fluid pressure from the pipeline is fed so that the housing and extension members are forced axially towards one another the balance of pressure prevailing in the pipeline. Additionally a shear disk is mounted between the housing and extension member to retain these two parts against relative axial movement. The shear disk is, however, designed to rupture at a predetermined load such that axial separation is prevented below that level and is permitted above that level.
While such a construction is satisfactory in theory, it has never proved fully satisfactory in practice because it has never been possible to overcome the problems of hydrostatic pressure prevailing at the bottom of the sea or fully to balance the loads caused by the oil pressure in the pipeline.